1. Field of the Invention
The present invention relates to the measurement of dishing and disc shape profiling of magneto-optical discs and digital video (or versatile) discs.
2. Description of the Related Art
As examples of conventional methods for measuring the tilt and dishing shape of discs having photoreflective surfaces such as magneto-optical discs and digital video discs, there are those wherein a laser beam is irradiated onto the surface being measured and the displacement of the reflected laser beam is measured by position detection means.
Conventionally, the displacement from the regular reflection position which is that for the case in which the laser beam from the light source is perpendicularly incident on the measurement surface is measured, and the tilt is calculated from the relationships between this displacement and the geometrical distance between the reflection point and the light reception point.
Hereinbelow, a tilt measurement method according to the conventional art will be explained using the drawings.
FIG. 7 shows the case wherein no mechanical deformation, such as dishing or warping, is present on the disc which is the object of measurement, that is, the condition wherein a laser beam 5 is perpendicularly incident on the irradiated surface of the disc and is regularly reflected. In FIG. 7, 1 denotes laser beam emission means, 2 denotes a beam splitter, 3 denotes a disc which is an object of measurement and 4 denotes position detection means. Additionally, the distance between the reflective surface of the beam splitter 2 and the position detection means 4 is denoted L.sub.A, and the distance between the reflective surface of the beam splitter 2 and the irradiated surface of the disc 3 is denoted L.sub.B.
FIG. 8 shows the case wherein the surface of the disc 3 irradiated by the laser beam 5 is tilted by an angle .theta. with respect to a reference plane (the plane assuming that the laser beam 5 is perpendicularly incident, illustrated by the dashed line). This tilt .theta. is caused by mechanical deformations such as dishing and warping of the disc 3. A simple geometrical analysis will show that when the tilt of the disc has an angle .theta. with respect to the reference plane, the laser beam 5 will be reflected at an angle equal to two times .theta. with respect to the reference plane.
The path of the laser beam 5 which has been reflected and returned is changed by approximately 90 degrees by the beam splitter 2 and is incident on the light receiving surface of the position detection means 4.
When the position of the incident reflected beam is displaced from the regular reflection position, the position detection means 4 outputs a voltage having a value proportional to the displacement.
When the displacement X of the reflected laser beam is specified by the output voltage of the position detection means 4, the tilt (dishing) angle of the irradiated surface is generally defined to be 2.theta., and is determined by the following equation. ##EQU1##
Generally, the value of 2.theta. is measured over the entire surface of the disc, and the maximum value is defined to be the tilt. The angle .theta. represents the actual angle by which the disc is tilted with respect to the reference plane. Alternatively, if 2.theta. in equation (1) is given in radians and (L.sub.A +L.sub.B)&gt;&gt;X, then there is no problem in using the approximation 2.theta.=(X/(L.sub.A +L.sub.B) for practical purposes.
In order to measure the tilt 2.theta. over the entire area of the disc 3, the relative positions of the laser beam emission means 1 and the position detection means 4 shown in FIG. 7 are fixed, and the output of the position detection means 4 is continuously read or recorded while the incident position of the laser beam 5 is moved along the radial direction of the disc 3 while rotating the disc 3.
The above-mentioned relative movement method allows the entire surface of a disc to be scanned, for example, by affixing a laser beam emission means and a position detection means and providing a mechanism for moving a disc horizontally while rotating. Alternatively, it is possible to have the disc be stationary and simply rotating, while the laser beam emission means and the position detection means are moved along the radial direction of the disc. As another method, a method for measuring the dishing of a disc based on the static electricity capacity may be considered, but this method has a drawback in that measurements must be made while precisely tracing the grooves contained in the photoreflective recording layer, so that an extremely long time is required. This drawback is apparent when in-line inspections are performed during continuous production, especially when complete inspections are performed.
As examples of methods for displaying the results obtained by these methods, there are methods wherein time is represented by a horizontal axis and the change in the output voltage from the position detection means 4 is simply displayed over time; alternatively, there are methods wherein the tilt is calculated from the dimensional constants determined by the geometrical arrangement of the disc 3 and the position detection means 4, and a variable representing the position on the disc such as the angle or the position along the circumferential direction is represented by the horizontal axis while the tilt is represented by the vertical axis.
However, even with these methods, there are problems in that the measurement results do not allow for a direct grasp on what type of shape the disc actually has and which parts are warped in which direction.
This is considered to be due to the fact that the physical quantities obtained by the measurements are the abstract quantities of "angles", and are no more than the enumeration of data for each point on the disc surface.
In consideration of these situations, the subject of the present invention is to offer a method and device for measuring the dishing of discs based on the tilt data of the discs, and further to offer a method and device which determines their approximate three-dimensional shapes by means of numerical operations and provides displays thereof.